Association of myopia and astigmatism with postoperative ocular high order aberration after small incision lenticule extraction

Scientific findings have reported a significant increase in the prevalence of myopia worldwide over the past few years [1]. It is estimated that by 2050, more than 4.76 billion people globally will have myopia, and roughly 940 million of them will have myopia of -5.00 diopter (D) or greater [2]. The prevalence of this condition has resulted in significant anguish in patients’ everyday experiences. Therefore, corneal refractive surgery has emerged as an effective solution to this problem [3, 4]. Small incision lenticule extraction (SMILE) is a modern technique for refractive surgery that was first described by Sekundo et al. in 2008 [5]. Following this, many studies have reported an outstanding improvement in visual acuity after SMILE. It offers the advantages of a convenient surgical procedure, stable postoperative recovery, and excellent treatment results. However, a significant proportion of patients still expressed dissatisfaction with their visual quality after undergoing SMILE. The post-SMILE patient group experienced an increasing number of visual problems, which became an important factor in evaluating the effectiveness of modern refractive surgery [6].

The increasing advancement and popularity of visual analysis technologies have highlighted higher-order aberrations (HOA) as the main factor affecting vision quality, particularly night vision symptoms [7]. This pertains to a type of optical distortion that occurs in the third order or above and cannot be corrected by wearing glass. It includes different types of aberration, such as spherical aberration (SA), coma aberration (CA), and trefoil aberration (TA). Studies have suggested that this anomaly may be linked to factors such as age, intraocular pressure (IOP), race, or refractive error [8‐11]. Research on refractive surgery has consistently shown that corneal refractive surgery, including SMILE, can induce HOA. In a study conducted by Sekundo et al., an increase in total HOA (tHOA) was observed after SMILE [12]. However, Kwak et al. noted that SMILE does not have a substantial impact on SA [13]. Therefore, no definitive result has been reached regarding the impact of myopia and astigmatism after SMILE on HOA [12‐15]. This study aimed to examine the relationship between the severity of myopia/astigmatism and the development of postoperative ocular HOA after SMILE, and to identify potentially relevant factors. Ultimately, the goal was to provide recommendations for reducing the induction of postoperative HOA and enhancing the visual quality after SMILE.

Most studies have shown a direct relationship between the degree of myopia and induction of HOA caused by SMILE [17‐20]. Higher levels of myopia result in thicker lenticules being cut, reducing corneal thickness and stability, causing excessive corneal deformation and increasing HOA. At the same time, the greater the degree of ablation, the greater the refractive difference between the unablated and ablated areas, which also causes HOA induction [18]. Similar findings were observed in this study, but there were still some differences compared to other studies regarding different types of HOA induction. For example, Li et al. [21] observed that in high myopia, both SA and CA induction increased with myopia severity, although SA induction varied differently from the preoperative myopic and CA curves. They believed that the higher the degree of myopia, the thicker the lenticule that needs to be removed, which may cause significant changes in corneal anterior asphericity. Subsequently, Kwon et al. [22] proposed a prediction that corneal flattening may produce a large amount of ocular SA; however, it should be noted that there is a nonlinear relationship between SA induction and myopia, which may be related to differences in biomechanical properties at different depths of the cornea. Jin et al. [23] observed that SA increases in the anterior corneal surface and whole cornea after SMILE. Moreover, SA induction in the high myopia group was significantly higher than that in the mild to moderate myopia group, while the increase in vertical CA also significantly exceeded that in the mild to moderate myopia group, which was consistent with the findings of Chen and Wu et al.[17, 18], and the pattern of change between vertical CA and myopia was consistent with the findings of this study. However, this study did not observe an increase in the postoperative SA with increasing degrees of myopia. The results could be affected by the selection of smaller pupil diameters or the lack of comparison of differences in HOA before and after surgery; therefore, the results may be influenced by the impact of myopia on HOA (some studies have shown that HOA is higher in myopic eyes than in other refractive states [24, 25]). It can be speculated that myopia severity likely influences the level of HOA after surgery. The increase in HOA was associated with the deterioration of corneal biomechanical stability and flatness, whereas the increase in CA may be due to the increase in ablation eccentricity caused by the increase in myopia.(Studies on HOA after SMILE are summarized in Supplementary Table 1).

Some researchers have studied the relationship between the degree of astigmatism and induction of HOA, but no consensus [26‐28]. Zhong et al. [15] observed that there was no significant difference in the induction of tHOA, SA, or CA between high and low astigmatism groups, which is consistent with the findings of Liu et al. [29] and Jun et al. [27] However, Huang et al. [28] confirmed that in eyes with large ablation eccentricity, the values of induced CA and SA were higher in the high astigmatism group than in the low astigmatism group, which was similar to the results of this study. Huang et al. further pointed out that for patients with high astigmatism, when the eccentricity of fixation exceeds 0.20 mm, the sensitivity between the significant induction of CA and the eccentric distance after SMILE increased significantly [28]. Subsequently, Lee et al. [30] observed that in cases where the eccentric distance of fixation exceeded 0.335 mm, aberrations exhibited similar significant differences. Therefore, if the ablation eccentricity exceeds a reasonable range, there will be a significant correlation between eccentricity and induced aberrations. However, other studies have reached conclusions inconsistent with the views of this study. Ding et al. [31] confirmed that regardless of the astigmatism intensity, there was no direct relationship between induced aberrations and optical zone eccentricity. Liu et al. [32] found that SA induction in the high astigmatism group was smaller than that in the non-astigmatism group, which they speculated was related to its larger functional optical zone (FOZ). The size of the FOZ may affect the effect of fixation eccentricity on induced CA, as the shape of the FOZ in high astigmatism is elliptical and has a different axis [30]. It can be understood that a higher astigmatism correction results in a larger FOZ, and the increased FOZ reduces the induction of SA and mitigates the induction of CA by eccentricity. The presence of eccentricity and counteracting effects brought about by the optical zone ultimately merge to form the degree of induction of HOA; thus, the degree of astigmatism was not significantly correlated with the induction of SA and CA. Although this study still found a link between postoperative tHOA, TA, and the degree of astigmatism, this may be due to postoperative corneal irregularities or TA inherent in astigmatism [33]. The lack of a link between postoperative SA and CA, and the degree of astigmatism in this study may be explained by the FOZ theory described above.

Total eye and corneal HOA showed more consistent behavior in most analyses, although internal HOA did not exhibit a significant association with parameters, such as myopia and astigmatism, in the majority of cases. This implies that the internal HOA is relatively stable, which may be another HOA compensation mechanism besides the posterior surface of the cornea [34]. Meanwhile, the analysis of internal HOA in this study was more consistent with the analysis of the posterior corneal surface in the study by Jin et al. [20]. They found that the HOA of the posterior corneal surface did not change with the degree of myopia and that it was stable in compensating for HOA before and after SMILE.

Further subgroup analysis revealed that there was almost no significant correlation between postoperative HOA (only vertical TA) and low myopia, but the opposite was true when moderate to high myopia was corrected. This seems to reflect that myopia is more likely to cause higher postoperative HOA when corrected to a higher degree. However, correction for low myopia is relatively safe. As shown in Fig. 3, the degree of myopia correction and the increase in postoperative HOA occured at a certain “node”, which may be due to the difficulty of compensating for internal HOA caused by higher degrees of myopia correction, or it may be related to corneal biomechanical stability: when correction leads to a thin residual corneal stroma bed thickness, reaching a critical value, it may be difficult to withstand IOP, which may be reflected preferentially in HOA [35]. Astigmatism was significantly associated with postoperative tHOA in both moderate/high and low astigmatism, although the former was associated with more types of HOA in addition to tHOA. The mechanism for this difference is different from myopia, and the increase in postoperative HOA may be related to a significant decrease in cutting eccentricity and corneal unevenness due to the correction of a higher degree of astigmatism. However, the highest degree of astigmatism also differed more from the degree of myopia in this study (the highest astigmatism degree was only -3D). Figure 3 shows that the fitting curve between the degree of astigmatism and postoperative tHOA has an approximately linear relationship. This may imply that there are differences in the mechanisms by which astigmatism affects SMILE, as opposed to myopia. However, correcting higher degrees of astigmatism does indeed lead to more HOA, which suggests that correcting higher degrees of astigmatism is less safe than correcting lower astigmatism.

Postoperative corneal lenticule thickness, postoperative UDVA, postoperative Cyl, postoperative Km, and postoperative CCT were all variables that have been shown to correlate with postoperative HOA. Results from the studies by Feng et al. [36] and Hu et al. [37] indicated a strong relationship between postoperative CCT and HOA after laser-assisted in situ keratomileusis (LASIK). They speculated that this could be related to worse corneal biomechanical properties after surgery, but they also believed that HOA induced by preoperative CCT could affect postoperative HOA. As some researchers have argued otherwise, more studies were needed to determine whether CCT and HOA are correlated [38, 39]. Decreased visual quality was generally associated with increased HOA, as Miller et al. [40] found that decreased BCVA was associated with higher levels of SA, but not CA. Namba et al. [41] reported an association between increased HOA and deterioration in visual function, which was similar to the results of this study. The increases in tHOA, CA, and SA should all decrease visual acuity, although the association between postoperative SA and HOA may not have been evident in this study because of pupillary factors. Other parameters, such as lenticule thickness and postoperative Km and Cyl, were determined based on the degree of myopia and astigmatism. In addition, this study did not find many associations between postoperative HOA and age, postoperative IOP, eyes, postoperative DS, or DC. Although some scholars have proposed an association between age, IOP, and gender with HOA [37, 42, 43], further research is needed to investigate its impact on introducing HOA after SMILE.

This study was the first to examine the changes in HOA after SMILE for myopia and astigmatism as well as to examine the factors associated with postoperative HOA. However, this study has some limitations. First, this study examined the pupil diameter (4 mm) under normal light conditions. Since a larger pupil diameter is believed to be associated with a higher HOA, future studies can broaden the investigation of pupil size to investigate the variations in HOA under dark conditions. Second, for bilaterally treated patients, there may be a correlation between the two eyes of one patient, which is a common mistake in ophthalmology research because the overall variance of a sample of measurements from both eyes is likely to be underestimated, increasing the risk of a type 1 error [44]. Finally, since this study focused on the association between postoperative factors and HOA, factors such as the focusing method and astigmatism axis adjustment that would significantly affect postoperative HOA were not explored [45, 46].

This study showed that an increase in the degree of myopia and astigmatism was associated with a higher degree of postoperative HOA after SMILE; however, the increase in HOA was not linearly correlated with the increase in myopia. Although there was a critical value, the degree of astigmatism showed a similar linear correlation. This finding suggested that SMILE can maintain excellent postoperative visual experience, at least in the case of mild myopia. The level of HOA after SMILE was associated with many factors; however, its specific clinical significance requires further specialized research. Therefore, postoperative HOA was affected by multiple factors, and more research on HOA induction and its causes in the future will be able to further explore the relevant findings of this study.